Electrochromic device having a self-cleaning hydrophilic coating

ABSTRACT

An electrochromic device is disclosed having a self-cleaning, hydrophilic optical coating. The electrochromic device preferably forms an external rearview mirror for a vehicle. The coating preferably includes alternating layers of a photocatalytic material having a high index of refraction and a hydrophilic material having a low refractive index. More specifically, the coating includes a first layer having a high refractive index, a second layer having a low refractive index, a third layer of titanium dioxide, and a fourth layer of silicon dioxide provided as an outermost layer. The disclosed optical coating exhibits a reflectance at the front surface of the reflective element that is less than about 20 percent, and has sufficient hydrophilic properties such that water droplets on a front surface of the optical coating exhibit a contact angle of less than about 20°.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 09/435,266 filed on Nov. 5, 1999 U.S. Pat. No. 6,193,378, which claims the benefit of U.S. Provisional Application No. 60/141,080, filed Jun. 25, 1999, both of which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention generally relates to electrochromic devices, and more specifically relates to electrochromic rearview mirrors of a vehicle.

To enable water droplets and mist to be readily removed from the windows of a vehicle, the windows are typically coated with a hydrophobic material that causes the water droplets to bead up on the outer surface of the window. These water beads are then either swept away by windshield wipers or are blown off the window as the vehicle moves.

It is equally desirable to clear external rearview mirrors of water. However, if a hydrophobic coating is applied to the external mirrors, the water beads formed on their surfaces cannot be effectively blown off since such mirrors are relatively shielded from direct airflow resulting from vehicle movement. Thus, water droplets or beads that are allowed to form on the surface of the mirrors remain on the mirror until they evaporate or grow in size until they fall from their own weight. These water droplets act as small lenses and distort the image reflected to the driver. Further, when the water droplets evaporate, water spots are left on the mirror, which are nearly as distracting as the water droplets that left the spots. In fog or high humidity, mist forms on the surfaces of the external mirrors. Such a mist can be so dense that it effectively renders the mirrors virtually unusable.

In an attempt to overcome the above-noted problems, mirror manufacturers have provided a hydrophilic coating on the outer surface of the external mirrors. See U.S. Pat. No. 5,594,585. One such hydrophilic coating includes a single layer of silicon dioxide (SiO₂). The SiO₂ layer is relatively porous. Water on the mirror is absorbed uniformly across the surface of the mirror into the pores of the SiO₂ layer and subsequently evaporates leaving no water spots. One problem with such single layer coatings of SiO₂ is that oil, grease, and other contaminants can also fill the pores of the SiO₂ layer. Many such contaminants, particularly hydrocarbons like oil and grease, do not readily evaporate and hence clog the pores of the SiO₂ layer. When the pores of the SiO₂ layer become clogged with car wax, oil, and grease, the mirror surface becomes hydrophobic and hence the water on the mirror tends to bead leading to the problems noted above.

A solution to the above problem pertaining to hydrophilic layers is to form the coating of a relatively thick layer (e.g., about 1000-3000 Å or more) of titanium dioxide (TiO₂). See European Patent Application Publication No. EPO 816 466 A1. This coating exhibits photocatalytic properties when exposed to ultraviolet (UV) radiation. More specifically, the coating absorbs UV photons and, in the presence of water, generates highly reactive hydroxyl radicals that tend to oxidize organic materials that have collected in its pores or on its surface. Consequently, hydrocarbons, such as oil and grease, that have collected on the mirror are converted to carbon dioxide (CO₂) and hence are eventually removed from the mirror whenever UV radiation impinges upon the mirror surface. This particular coating is thus a self-cleaning hydrophilic coating.

One measure of the hydrophilicity of a particular coating is to measure the contact angle that the sides of a water drop form with the surface of the coating. An acceptable level of hydrophilicity is present in a mirror when the contact angle is less than about 30°, and more preferably, the hydrophilicity is less than about 20°. The above self-cleaning hydrophilic coating exhibits contact angles that decrease when exposed to UV radiation as a result of the self-cleaning action and the hydrophilic effect of the coating. The hydrophilic effect of this coating, however, tends to reverse over time when the mirror is not exposed to UV radiation.

The above self-cleaning hydrophilic coating can be improved by providing a film of about 150 to 1000 Å of SiO₂ on top of the relatively thick TiO₂ layer. See U.S. Pat. No. 5,854,708. This seems to enhance the self-cleaning nature of the TiO₂ layer by reducing the dosage of UV radiation required and by maintaining the hydrophilic effect of the mirror over a longer period of time after the mirror is no longer exposed to UV radiation.

While the above hydrophilic coatings work well on conventional rearview mirrors having a chrome or silver layer on the rear surface of a glass substrate, they have not been considered for use on electrochromic mirrors for several reasons. A first reason is that many of the above-noted hydrophilic coatings introduce colored double images and increase the low-end reflectivity of the electrochromic mirror. For example, commercially available, outside electrochromic mirrors exist that have a low-end reflectivity of about 10 percent and a high-end reflectivity of about 50 to 65 percent. By providing a hydrophilic coating including a material such as TiO₂, which has a high index of refraction, on a glass surface of the mirror, a significant amount of the incident light is reflected at the glass/TiO₂ layer interface regardless of the variable reflectivity level of the mirror. Thus, the low-end reflectivity would be increased accordingly. Such a higher low-end reflectivity obviously significantly reduces the range of variable reflectance the mirror exhibits and thus reduces the effectiveness of the mirror in reducing annoying glare from the headlights of rearward vehicles.

Another reason that the prior hydrophilic coatings have not been considered for use on electrochromic elements is that they impart significant coloration problems. Coatings such as those having a 1000 Å layer of TiO₂ covered with a 150 Å layer of SiO₂, exhibit a very purple hue. When used in a conventional mirror having chrome or silver applied to the rear surface of a glass element, such coloration is effectively reduced by the highly reflective chrome or silver layer, since the color neutral reflections from the highly reflective layer overwhelm the coloration of the lower reflectivity, hydrophilic coating layer. However, if used on an electrochromic element, such a hydrophilic coating would impart a very objectionable coloration, which is made worse by other components in the electrochromic element that can also introduce color.

Due to the problems associated with providing a hydrophilic coating made of TiO₂ on an electrochromic mirror, manufacturers of such mirrors have opted to not use such hydrophilic coatings. As a result, electrochromic mirrors suffer from the above-noted adverse consequences caused by water drops and mist.

SUMMARY OF THE INVENTION

Accordingly, it is an aspect of the present invention to solve the above problems by providing a hydrophilic coating suitable for use on an electrochromic device, particularly for an electrochromic mirror. To achieve these and other aspects and advantages, a rearview mirror according to the present invention comprises an electrochromic mirror element having a reflectivity that may be varied in response to an applied voltage so as to exhibit at least a high reflectance state and low reflectance state, and a hydrophilic optical coating applied to a front surface of the electrochromic mirror element. The rearview mirror preferably exhibits a reflectance of less than 20 percent in said low reflectance state, and also preferably exhibits a C* value less than about 20 in both said high and low reflectance states so as to exhibit substantial color neutrality.

These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 a front perspective view of an external electrochromic rearview mirror assembly constructed in accordance with the present invention; and

FIG. 2 is a s section of the external electrochromic rearview mirror assembly shown in FIG. 1 along line II—II.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an external electrochromic rearview mirror assembly 10 constructed in accordance with the present invention. As shown, mirror assembly 10 generally includes a housing 15 and a mirror 20 movably mounted in housing 15. Housing 15 may have any conventional structure suitably adapted for mounting assembly 10 to the exterior of a vehicle.

FIG. 2 shows an exemplary construction of mirror 20. As broadly described herein, mirror 20 includes a reflective element 100 having a reflectivity that may be varied in response to an applied voltage and an optical coating 130 applied to a front surface 112 a of reflective element 100. Reflective element 100 preferably includes a first (or front) element 112 and a second (or rear) element 114 sealably bonded in spaced-apart relation to define a chamber. Front element 112 has a front surface 112 a and a rear surface 112 b, and rear element 114 has a front surface 114 a and a rear surface 114 b. For purposes of further reference, front surface 112 a of front element 112 shall be referred to as the first surface, rear surface 112 b of front element 112 shall be referred to as the second surface, front surface 114 a of rear element 114 shall be referred to as the third surface, and rear surface 114 b of rear element 114 shall be referred to as the fourth surface of reflective element 100. Preferably, both elements 112 and 114 are transparent and are sealably bonded by means of a seal member 116.

Reflective element 100 also includes a transparent first electrode 118 carried on one of second surface 112 b and third surface 114 a, and a second electrode 120 carried on one of second surface 112 b and third surface 114 a. Second electrode 120 may be reflective or transflective, or a separate reflector 122 may be provided on fourth surface 114 b of mirror 100 in which case electrode 120 would be transparent. Preferably, however, second electrode 120 is reflective or transflective and the layer referenced by numeral 122 is an opaque layer or omitted entirely. Reflective element 100 also includes an electrochromic medium 124 contained in the chamber in electrical contact with first and second electrodes 118 and 120.

Electrochromic medium 124 includes electrochromic anodic and cathodic materials that can be grouped into the following categories:

(i) Single layer—the electrochromic medium is a single layer of material which may include small nonhomogeneous regions and includes solution-phase devices where a material is contained in solution in the ionically conducting electrolyte and remains in solution in the electrolyte when electrochemically oxidized or reduced. Solution-phase electroactive materials may be contained in the continuous solution phase of a cross-linked polymer matrix in accordance with the teachings of U.S. Pat. No. 5,928,572, entitled “IMPROVED ELECTROCHROMIC LAYER AND DEVICES COMPRISING SAME” or International Patent Application No. PCT/US98/05570 entitled “ELECTROCHROMIC POLYMERIC SOLID FILMS, MANUFACTURING ELECTROCHROMIC DEVICES USING SUCH SOLID FILMS, AND PROCESSES FOR MAKING SUCH SOLID FILMS AND DEVICES.”

At least three electroactive materials, at least two of which are electrochromic, can be combined to give a pre-selected color as described in U.S. Pat. No. 6,020,987 entitled “ELECTROCHROMIC MEDIUM CAPABLE OF PRODUCING A PRE-SELECTED COLOR.”

The anodic and cathodic materials can be combined or linked by a bridging unit as described in International Application No. PCT/WO97/EP498 entitled “ELECTROCHROMIC SYSTEM.” It is also possible to link anodic materials or cathodic materials by similar methods. The concepts described in these applications can further be combined to yield a variety of electrochromic materials that are linked.

Additionally, a single layer medium includes the medium where the anodic and cathodic materials can be incorporated into the polymer matrix as described in International Application No. PCT/WO98/EP3862 entitled “ELECTROCHROMIC POLYMER SYSTEM” or International Patent Application No. PCT/US98/05570 entitled “ELECTROCHROMIC POLYMERIC SOLID FILMS, MANUFACTURING ELECTROCHROMIC DEVICES USING SUCH SOLID FILMS, AND PROCESSES FOR MAKING SUCH SOLID FILMS AND DEVICES.”

Also included is a medium where one or more materials in the medium undergoes a change in phase during the operation of the device, for example, a deposition system where a material contained in solution in the ionically conducting electrolyte, which forms a layer or partial layer on the electronically conducting electrode when electrochemically oxidized or reduced.

(ii) Multilayer—the medium is made up in layers and includes at least one material attached directly to an electronically conducting electrode or confined in close proximity thereto, which remains attached or confined when electrochemically oxidized or reduced. Examples of this type of electrochromic medium are the metal oxide films, such as tungsten oxide, iridium oxide, nickel oxide, and vanadium oxide. A medium, which contains one or more organic electrochromic layers, such as polythiophene, polyaniline, or polypyrrole attached to the electrode, would also be considered a multilayer medium.

In addition, the electrochromic medium may also contain other materials, such as light absorbers, light stabilizers, thermal stabilizers, antioxidants, thickeners, or viscosity modifiers.

Because reflective element 100 may have essentially any structure, the details of such structures are not further described. Examples of preferred electrochromic mirror constructions are disclosed in U.S. Pat. No. 4,902,108, entitled “SINGLE-COMPARTMENT, SELF-ERASING, SOLUTION-PHASE ELECTROCHROMIC DEVICES SOLUTIONS FOR USE THEREIN, AND USES THEREOF,” issued Feb. 20, 1990, to H. J. Byker; Canadian Patent No. 1,300,945, entitled “AUTOMATIC REARVIEW MIRROR SYSTEM FOR AUTOMOTIVE VEHICLES,” issued May 19, 1992, to J. H. Bechtel et al.; U.S. Pat. No. 5,128,799, entitled “VARIABLE REFLECTANCE MOTOR VEHICLE MIRROR,” issued Jul. 7, 1992, to H. J. Byker; U.S. Pat. No. 5,202,787, entitled “ELECTRO-OPTIC DEVICE,” issued Apr. 13, 1993, to H. J. Byker et al.; U.S. Pat. No. 5,204,778, entitled “CONTROL SYSTEM FOR AUTOMATIC REARVIEW MIRRORS,” issued Apr. 20, 1993, to J. H. Bechtel; U.S. Pat. No. 5,278,693, entitled “TINTED SOLUTION-PHASE ELECTROCHROMIC MIRRORS,” issued Jan. 11, 1994, to D. A. Theiste et al.; U.S. Pat. No. 5,280,380, entitled “UV-STABILIZED COMPOSITIONS AND METHODS,” issued Jan. 18, 1994, to H. J. Byker; U.S. Pat. No. 5,282,077, entitled “VARIABLE REFLECTANCE MIRROR,” issued Jan. 25, 1994, to H. J. Byker; U.S. Pat. No. 5,294,376, entitled “BIPYRIDINIUM SALT SOLUTIONS,” issued Mar. 15, 1994, to H. J. Byker; U.S. Pat. No. 5,336,448, entitled “ELECTROCHROMIC DEVICES WITH BIPYRIDINIUM SALT SOLUTIONS,” issued Aug. 9, 1994, to H. J. Byker; U.S. Pat. No. 5,434,407, entitled “AUTOMATIC REARVIEW MIRROR INCORPORATING LIGHT PIPE,” issued Jan. 18, 1995, to F. T. Bauer et al.; U.S. Pat. No. 5,448,397, entitled “OUTSIDE AUTOMATIC REARVIEW MIRROR FOR AUTOMOTIVE VEHICLES,” issued Sep. 5, 1995, to W. L. Tonar; U.S. Pat. No. 5,451,822, entitled “ELECTRONIC CONTROL SYSTEM,” issued Sep. 19, 1995, to J. H. Bechtel et al.; U.S. Pat. No. 5,818,625, entitled “ELECTROCHROMIC REARVIEW MIRROR INCORPORATING A THIRD SURFACE METAL REFLECTOR,” by Jeffrey A. Forgette et al. ; and U.S. patent application No. 09/158,423, entitled “IMPROVED SEAL FOR ELECTROCHROMIC DEVICES,” filed on Sep. 21, 1998. Each of these patents and the patent application are commonly assigned with the present invention and the disclosures of each, including the references contained therein, are hereby incorporated herein in their entirety by reference.

If the mirror assembly includes a signal light, display, or other indicia behind the reflective electrode or reflective layer of reflective element 100, reflective element 100 is preferably constructed as disclosed in commonly assigned U.S. patent application Ser. No. 09/311,955, entitled “ELECTROCHROMIC REARVIEW MIRROR INCORPORATING A THIRD SURFACE METAL REFLECTOR AND A DISPLAY/SIGNAL LIGHT,” filed on May 14, 1999, by W. L. Tonar et al., the disclosure of which is incorporated herein by reference. If reflective element 100 is convex or aspheric, as is common for passenger-side external rearview mirrors as well as external driver-side rearview mirrors of cars in Japan and Europe, reflective element 100 may be made using thinner elements 112 and 114 while using a polymer matrix in the chamber formed therebetween as is disclosed in commonly assigned U.S. Pat. No. 5,940,201 entitled “AN ELECTROCHROMIC MIRROR WITH TWO THIN GLASS ELEMENTS AND A GELLED ELECTROCHROMIC MEDIUM,” filed on Apr. 2, 1997. The entire disclosure, including the references contained therein, of this U.S. patent is incorporated herein by reference. The addition of the combined reflector/electrode 120 onto third surface 114 a of reflective element 100 further helps remove any residual double imaging resulting from the two glass elements being out of parallel. The electrochromic element of the present invention is preferably color neutral. In a color neutral electrochromic element, the element darkens to a gray color, which is more ascetically pleasing than any other color when used in an electrochromic mirror. U.S. Pat. No. 6,020,987, entitled “ELECTROCHROMIC MEDIUM CAPABLE OF PRODUCING A PRE-SELECTED COLOR” discloses electrochromic media that are perceived to be gray throughout their normal range of operation. The entire disclosure of this patent is hereby incorporated herein by reference. U.S. patent application No. 09/311,955 entitled “ELECTROCHROMIC REARVIEW MIRROR INCORPORATING A THIRD SURFACE METAL REFLECTOR AND A DISPLAY/SIGNAL LIGHT” discloses additional electrochromic mirrors that exhibit substantial color neutrality while enabling displays to be positioned behind the reflective surface of the electrochromic mirror. The entire disclosure of this application is hereby incorporated herein by reference.

In addition to reflective element 100, mirror 20 further includes an optical coating 130. Optical coating 130 is a self-cleaning hydrophilic optical coating. Optical coating 130 preferably exhibits a reflectance at first surface 112 a of reflective element 100 that is less than about 20 percent. If the reflectance at first surface 112 a is greater than about 20 percent, noticeable double-imaging results, and the range of variable reflectance of reflective element 100 is significantly reduced. The electrochromic mirror as a unit should have a reflectance of less than about 20 percent in its lowest reflectance state, and more preferably should have a reflectance of less than about 10 percent.

Optical coating 130 also is preferably sufficiently hydrophilic such that water droplets on a front surface of coating 130 exhibit a contact angle of less than about 30°, more preferably less than about 20°, and most preferably less than about 10°. If the contact angle is greater than about 30°, the coating 130 exhibits insufficient hydrophilic properties to prevent distracting water beads from forming. Optical coating 130 should also exhibit self-cleaning properties whereby the hydrophilic properties may be restored following exposure to UV radiation.

In one embodiment, optical coating 130 includes at least four layers of alternating high and low refractive index. Specifically, as shown in FIG. 2, optical coating 130 includes, in sequence, a first layer 132 having a high refractive index, a second layer 134 having a low refractive index, a third layer 136 having a high refractive index, and a fourth layer 138 having a low refractive index. Preferably, third layer 136 is made of a photocatalytic material, and fourth layer 138 is made of a material that will enhance the hydrophilic properties of the photocatalytic layer 136 by generating hydroxyl groups on its surface. Suitable hydrophilic enhancement materials include SiO₂ and Al₂O₃, with SiO₂ being most preferred. Suitable photocatalytic materials include TiO₂, ZnO, ZnO₂, SnO₂, ZnS, CdS, CdSe, Nb₂O₅, KTaNbO₃, KTaO₃, SrTiO₃, WO₃, Bi2O₃, Fe2O₃, and GaP, with TiO₂ being most preferred. By making the outermost layers TiO₂ and SiO₂, coating 130 exhibits good self-cleaning hydrophilic properties similar to those obtained by the prior art hydrophilic coatings applied to conventional mirrors having a reflector provided on the rear surface of a single front glass element. Preferably, the thickness of the SiO₂ outer layer is less than about 800 Å. If the SiO₂ outer layer is too thick (e.g., more than about 1000 Å), the underlying photocatalytic layer will not be able to “clean” the SiO₂ hydrophilic outer layer, at least not within a short time period. The two additional layers (layers 132 and 134) are provided to reduce the undesirable reflectance levels at the front surface of reflective element 100. Preferably, layer 132 is made of a photocatalytic material and second layer 134 is made of a hydrophilic enhancement material so as to contribute to the hydrophilic and photocatalytic properties of the coating. Thus, layer 132 may be made of any one of the photocatalytic materials described above or mixtures thereof, and layer 134 may be made of any of the hydrophilic enhancement materials described above or mixtures thereof. Preferably layer 132 is made of TiO₂, and layer 134 is made of SiO₂.

An alternative technique to using a high index layer and low index layer between the glass and the layer that is primarily comprised of photocatalytic metal oxide (i.e., layer 136) to obtain all of the desired properties while maintaining a minimum top layer thickness of primarily silica is to use a layer, or layers, of intermediate index. This layer(s) could be a single material such as tin oxide or a mixture of materials such as a blend of titania and silica. Among the materials that have been modeled as potentially useful are blends of titania and silica, which can be obtained through sol-gel deposition as well as other means, and tin oxide. One can use a graded index between the glass and layer primarily composed of photocatalytic material as well.

Additionally, one can obtain roughly the same color and reflectance properties with a thinner top layer or possibly no top layer containing primarily silica if the index of the photocatalytic layer is lowered somewhat by blending materials, as would be the case, for example, for a titania and silica mixture deposited by sol-gel. The lower index of the titania and silica blend layer imparts less reflectivity, requires less compensation optically, and therefore allows for a thinner top layer. This thinner top layer should allow for more of the photocatalytic effect to reach surface contaminants.

The index of refraction of a titania film obtained from a given coating system can vary substantially with the choice of coating conditions and could be chosen to give the lowest index possible while maintaining sufficient amounts of anatase or rutile form in the film and demonstrating adequate abrasion resistance and physical durability. The lower index obtained in this fashion would yield similar advantages to lowering the index by mixing titania with a lower index material. Ron Willey, in his book “Practical Design and Production of Optical Thin Films,” Marcel Dekker, 1996, cites an experiment where temperature of the substrate, partial pressure of oxygen, and speed of deposition vary the index of refraction of the titania deposited from about n=2.1 to n=2.4.

Materials used for transparent second surface conductors are typically materials whose index of refraction is about 1.9 or greater and have their color minimized by using half wave thickness multiples or by using the thinnest layer possible for the application or by the use of one of several “non-iridescent glass structures.” These non-iridescent structures will typically use either a high and low index layer under the high index conductive coating (see, for example, U.S. Pat. No. 4,377,613 and U.S. Pat. No. 4,419,386 by Roy Gordon), or an intermediate index layer (see U.S. Pat. No. 4,308,316 by Roy Gordon) or graded index layer (see U.S. Pat. No. 4,440,822 by Roy Gordon).

Fluorine doped tin oxide conductors using a non-iridescent structure are commercially available from Libbey-Owens-Ford and are used as the second surface transparent conductors in most inside automotive electrochromic mirrors produced at the present time. The dark state color of devices using this second surface coating stack is superior to that of elements using optical half wave thickness indium tin oxide (ITO) when it is used as a second surface conductive coating. Drawbacks of this non-iridescent coating are mentioned elsewhere in this document. Hydrophilic and photocatalytic coating stacks with less than about 800 Å silica top layer, such as 1000 Å titania 500 Å silica, would still impart unacceptable color and/or reflectivity when used as a first surface coating stack in conjunction with this non-iridescent second surface conductor and other non-iridescent second surface structures, per the previous paragraph, that are not designed to compensate for the color of hydrophilic coating stacks on the opposing surface. Techniques would still need to be applied per the present embodiment at the first surface to reduce C* of the system in the dark state if these coatings were used on the second surface.

ITO layers typically used are either very thin (approximately 200-250 Å), which minimizes the optical effect of the material by making it as thin as possible while maintaining sheet resistances adequate for many display devices, or multiples of half wave optical thickness (about 1400 Å), which minimizes the overall reflectivity of the coating. In either case, the addition of photocatalytic hydrophilic coating stacks on opposing surfaces per the previous paragraph would create unacceptable color and/or reflectivity in conjunction with the use of these layer thicknesses of ITO used as the second surface conductor. Again, techniques would need to be applied per the present embodiment at the first surface to reduce the C* of the system in the dark state.

In somewhat analogous fashion, for modification of the first surface-coating stack to optimize the color and reflectivity of the system containing both first and second surface coatings, one can modify the second surface-coating stack to optimize the color of the system. One would do this by essentially creating a compensating color at the second surface in order to make reflectance of the system more uniform across the visible spectrum, while still maintaining relatively low overall reflectance. For example, the 1000 Å titania 500 Å silica stack discussed in several places within this document has a reddish-purple color due to having somewhat higher reflectance in both the violet and red portions of the spectrum than it has in the green. A second surface coating with green color, such as ¾ wave optical thickness ITO, will result in a lower C* value for the dark state system than a system with a more standard thickness of ITO of half wave optical thickness, which is not green in color. Additionally, one can modify thicknesses of layers or choose materials with somewhat different indices in the non-iridescent structures mentioned in order to create a compensating color second surface as well.

Another method of color compensating the first surface is through pre-selecting the color of the electrochromic medium in the dark state in accordance with the teachings of commonly assigned U.S. Pat. No. 6,020,987, entitled “ELECTROCHROMIC MEDIUM CAPABLE OF PRODUCING A PRE-SELECTED COLOR.” Again, by using first surface coatings of 1000 Å titania followed by 500 Å silica as an example, the following modification would assist in lowering the C* value of an electrochromic mirror when activated. If, in that case, the color of the electrochromic medium was selected so that it was less absorbing in the green region when activated, the higher reflection of green wavelengths of light from the third or fourth surface reflector of the element would help balance the reflection of the unit in the dark state.

Combinations of the aforementioned concepts for the first, second surface, and electrochromic medium are also potentially advantageous for the design.

It is known that the optical properties for a deposited film vary depending on deposition conditions that include partial pressure of oxygen gas, temperature of the substrate speed of deposition, and the like. In particular, the index of refraction for a particular set of parameters on a particular system will affect the optimum layer thicknesses for obtaining the optical properties being discussed.

The discussions regarding the photocatalytic and hydrophilic properties of titania and like photocatalytic materials and silica and like hydrophilic materials are generally applicable to layers of mixed materials as long as the mixtures retain the basic properties of photocatalytic activity and/or hydrophilicity. Abrasion resistance is also a major consideration in the outermost layer. EP 0816466A1 describes an abrasion resistant, photocatalytic, hydrophilic layer of silica blended titania, as well as a layer of tin oxide blended titania with similar properties. U.S. Pat. No. 5,755,867 describes photocatalytic blends of silica and titania obtained through use of these mixtures. These coatings would likely require modifications to change their optical properties suitable for use on an electrochromic device. The potential advantages of these optical property modifications to this invention are discussed further below.

In some variations of this invention, it may be preferable to include a layer of material between the substrate, especially if it is soda lime glass, and the photocatalytic layer(s) to serve as a barrier against sodium leaching in particular. If this layer is close to the index of refraction of the substrate, such as silica on soda lime glass, it will not affect the optical properties of the system greatly and should not be considered as circumventing the spirit of the invention with regards to contrasting optical properties between layers.

To expedite the evaporation of water on the mirror and prevent the freezing of thin films of water on the mirror, a heating element 122 may optionally be provided on the fourth surface 114 b of reflective element 100. Alternatively, one of the transparent front surface films could be formed of an electrically conductive material and hence function as a heater.

To illustrate the properties and advantages of the present invention, an example is provided below. The following illustrative example is not intended to limit the scope of the present invention but to illustrate its application and use. In this example, references are made to the spectral properties of an electrochromic mirror constructed in accordance with the parameters specified in the example. In discussing colors, it is useful to refer to the Commission Internationale de I'Eclairage's (CIE) 1976 CIELAB Chromaticity Diagram (commonly referred to as the L*a*b* chart). The technology of color is relatively complex, but a fairly comprehensive discussion is given by F. W. Billmeyer and M. Saltzman in Principles of Color Technology, 2nd Edition, J. Wiley and Sons Inc. (1981), and the present disclosure, as it relates to color technology and terminology, generally follows that discussion. On the L*a*b* chart, L* defines lightness, a* denotes the red/green value, and b* denotes the yellow/blue value. Each of the electrochromic media has an absorption spectra at each particular voltage that may be converted to a three-number designation, their L*a*b* values. To calculate a set of color coordinates, such as L*a*b* values, from the spectral transmission or reflectance, two additional items are required. One is the spectral power distribution of the source or illuminant. The present disclosure uses CIE Standard Illuminant D₆₅. The second item needed is the spectral response of the observer. The present disclosure uses the 2-degree CIE standard observer. The illuminant/observer combination used is represented as D₆₅/2 degree. Many of the examples below refer to a value Y from the 1931 CIE Standard since it corresponds more closely to the reflectance than L*. The value C*, which is also described below, is equal to the square root of (a*)²+(b*)², and hence, provides a measure for quantifying color neutrality. To obtain an electrochromic mirror having relative color neutrality, the C* value of the mirror should be less than 20. Preferably, the C* value is less than 15, and more preferably is less than about 10.

EXAMPLE

Two identical electrochromic mirrors were constructed having a rear element made with 2.2 mm thick glass with a layer of chrome applied to the front surface of the rear element and a layer of rhodium applied on top of the layer of chrome using vacuum deposition. Both mirrors included a front transparent element made of 1.1 mm thick glass, which was coated on its rear surface with a transparent conductive ITO coating of ½ wave optical thickness. The front surfaces of the front transparent elements were covered by a coating that included a first layer of 200 Å thick TiO₂, a second layer of 250 Å thick SiO₂, a third layer of 1000 Å TiO₂, and a fourth layer of 500 Å thick SiO₂. For each mirror, an epoxy seal was laid about the perimeter of the two coated glass substrates except for a small port used to vacuum fill the cell with electrochromic solution. The seal had a thickness of about 137 microns maintained by glass spacer beads. The elements were filled with an electrochromic solution including propylene carbonate containing 3 percent by weight polymethylmethacrylate, 30 mM Tinuvin P (UV absorber), 38 mM N,N′-dioctyl-4, 4′bipyridinium bis(tetrafluoroborate), 27 mM 5,10-dihydrodimethylphenazine and the ports were then plugged with a UV curable adhesive. Electrical contact buss clips were electrically coupled to the transparent conductors.

In the high reflectance state (with no potential applied to the contact buss clips), the electrochromic mirrors had the following averaged values: L*=78.26, a*=−2.96, b*=4.25, C*=5.18, and Y=53.7. In the lowest reflectance state (with a potential of 1.2 V applied), the electrochromic mirrors had the following averaged values: L*=36.86, a*=6.59, b*=−3.51, C*=7.5, and Y=9.46. The average contact angle that a drop of water formed on the surfaces of the electrochromic mirrors after it was cleaned was 7°.

For purposes of comparison, two similar electrochromic mirrors were constructed, but without any first surface coating. These two mirrors had identical construction. In the high reflectance state, the electrochromic mirrors had the following averaged values: L*=78.93, a*=−2.37, b*=2.55, C*=3.48, and Y=54.81. In the lowest reflectance state, the electrochromic mirrors had the following averaged values: L*=29.46, a*=0.55, b*=−16.28, C*=16.29, and Y=6.02. As this comparison shows, the electrochromic mirrors having the inventive hydrophilic coating unexpectedly and surprisingly had better color neutrality than similarly constructed electrochromic mirrors not having such a hydrophilic coating. Additionally, the comparison shows that the addition of the hydrophilic coating does not appreciably increase the low-end reflectance of the mirrors.

The present invention thus provides a hydrophilic coating that not only is suitable for an electrochromic device, but actually improves the color neutrality of the device.

Although the example cited above uses a vacuum deposition technique to apply the coating, these coatings can also be applied by conventional sol-gel techniques. In this approach, the glass is coated with a metal alkoxide made from precursors such as tetra isopropyl titanate, tetra ethyl ortho silicate, or the like. These metal alkoxides can be blended or mixed in various proportions and coated onto glass usually from an alcohol solution after being partially hydrolyzed and condensed to increase the molecular weight by forming metal oxygen metal bonds. These coating solutions of metal alkoxides can be applied to glass substrates by a number of means such as dip coating, spin coating, or spray coating. These coatings are then fired to convert the metal alkoxide to a metal oxide typically at temperatures above 450° C. Very uniform and durable thin film can be formed using this method. Since a vacuum process is not involved, these films are relatively inexpensive to produce. Multiple films with different compositions can be built up prior to firing by coating and drying between applications. This approach can be very useful to produce inexpensive hydrophilic coatings on glass for mirrors, especially convex or aspheric mirrors that are made from bent glass. In order to bend the glass, the glass must be heated to temperatures above 550° C. If the sol-gel coatings are applied to the flat glass substrate before bending (typically on what will be the convex surface of the finished mirror), the coatings will fire to a durable metal oxide during the bending process. Thus, a hydrophilic coating can be applied to bent glass substrates for little additional cost. Since the majority of outside mirrors used in the world today are made from bent glass, this approach has major cost benefits. It should be noted that some or all of the coatings could be applied by this sol-gel process with the remainder of the coating(s) applied by a vacuum process, such as sputtering or E-beam deposition. For example, the first high index layer and low index layer of, for instance, TiO₂ and SiO₂ could be applied by a sol-gel technique and then the top TiO₂ and SiO₂ layer applied by sputtering. This would simplify the requirements of the coating equipment and yield cost savings. It is desirable to prevent migration of ions, such as sodium, from soda lime glass substrates into the photocatalytic layer. The sodium ion migration rate is temperature dependent and occurs more rapidly at high glass bending temperatures. A sol-gel formed silica or doped silica layer, for instance phosphorous doped silica, is effective in reducing sodium migration. This barrier underlayer can be applied using a sol-gel process. This silica layer could be applied first to the base glass or incorporated into the hydrophilic stack between the photocatalytic layer and the glass.

In general, the present invention is applicable to any electrochromic element including architectural windows and skylights, automobile windows, rearview mirrors, and sunroofs. With respect to rearview mirrors, the present invention is primarily intended for outside mirrors due to the increased likelihood that they will become foggy or covered with mist. Inside and outside rearview mirrors may be slightly different in configuration. For example, the shape of the front glass element of an inside mirror is generally longer and narrower than outside mirrors. There are also some different performance standards placed on an inside mirror compared with outside mirrors. For example, an inside mirror generally, when fully cleared, should have a reflectance value of about 70 percent to about 85 percent or higher, whereas the outside mirrors often have a reflectance of about 50 percent to about 65 percent. Also, in the United States (as supplied by the automobile manufacturers), the passenger-side mirror typically has a non-planar spherically bent or convex shape, whereas the driver-side mirror 111 a and inside mirror 110 presently must be flat. In Europe, the driver-side mirror 111 a is commonly flat or aspheric, whereas the passenger-side mirror 111 b has a convex shape. In Japan, both outside mirrors have a non-planar convex shape.

The fact that outside rearview mirrors are often non-planar raises additional limitations on their design. For example, the transparent conductive layer applied to the rear surface of a non-planar front element is typically not made of fluorine-doped tin oxide, which is commonly used in planar mirrors, because the tin oxide coating can complicate the bending process and it is not commercially available on glass thinner than 2.3 mm. Thus, such bent mirrors typically utilize a layer of ITO as the front transparent conductor. ITO, however, is slightly colored and adversely introduces blue coloration into the reflected image as viewed by the driver. The color introduced by an ITO layer applied to the second surface of the element may be neutralized by utilizing an optical coating on the first surface of the electrochromic element. To illustrate this effect, a glass element coated with a half wave thick ITO layer was constructed as was a glass element coated with a half wave thick ITO layer on one side and the hydrophilic coating described in the above example on the other side. The ITO-coated glass without the hydrophilic coating had the following properties: L*=37.09, a*=8.52, b*=−21.12, C*=22.82, and a first/second surface spectral reflectance of Y=9.58. By contrast, the ITO-coated glass that included the inventive hydrophilic coating of the above-described example exhibited the following properties: L*=42.02, a*=2.34, b*=−8.12, C*=8.51, and a first/second surface spectral reflectance of Y=12.51. As evidenced by the significantly reduced C* value, the hydrophilic coating serves as a color suppression coating by noticeably improving the coloration of a glass element coated with ITO. Because outside rearview mirrors are often bent and include ITO as a transparent conductor, the ability to improve the color of the front coated element by adding a color suppression coating to the opposite side of the bent glass provides many manufacturing advantages.

Other light attenuating devices, such as scattered particle displays or liquid crystal displays can also benefit from the application of these principles. In devices where the light attenuating layer is between two pieces of glass or plastic, the same basic constraints and solutions to those constraints will apply. The color and reflectivity of a first surface hydrophilic layer or layer stack can impart substantial color and reflectivity to the device in the darkened state even when this first surface layer stack does not appreciably affect the bright state characteristics. Adjustments to the first surface layer stack similar to those discussed for an electrochromic device will, therefore, affect the color and/or reflectivity of the darkened device advantageously. The same will apply to adjustments made to the second surface of the device or to the color of the darkening layer itself.

These principles can also be applied to devices such as insulated windows where the light attenuating device may be contained within a structure having, for example, an additional outer and inner pane of glass for insulation purposes. The hydrophilic coating would, in this case, need to be on the outside of the multilayer structure, but would affect the color of the darkened device similarly. The techniques discussed could be advantageously applied in this structure. Those familiar with the art can see that coatings could or would be transferred to a different surface or surfaces of the device because of the additional surfaces either available or interposing within the device.

The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and not intended to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the Doctrine of Equivalents. 

What is claimed is:
 1. An electro-optic device comprising: an electro-optic element having a variable transmittance, said electro-optic element having a front surface and a rear surface; and a self-cleaning hydrophilic optical coating disposed on one of the front and rear surfaces of said electro-optic element, said hydrophilic optical coating comprising alternating layers of a photocatalytic material having a high index of refraction and a hydrophilic material having a low refractive index.
 2. The electro-optic device as defined in claim 1, wherein said photocatalytic material is titanium dioxide.
 3. The electro-optic device as defined in claim 2, wherein said hydrophilic material is silicon dioxide.
 4. The electro-optic device as defined in claim 1, wherein said hydrophilic material is silicon dioxide.
 5. The electro-optic device as defined in claim 1, wherein a first layer of said coating adjacent the front surface of the electro-optic element is made of said photocatalytic material.
 6. The electro-optic device as defined in claim 1, wherein said electro-optic element includes: front and rear substrates sealably bonded together to form a chamber, each substrate including a front and a rear surface, the front surface of said front substrate serving as the front surface of said electrochromic element; an electrochromic medium contained in said chamber; a transparent first electrode carried on one of the front surface of said rear substrate and said rear surface of said front substrate; and a second electrode carried on the front surface of said rear substrate.
 7. The electro-optic device as defined in claim 6, wherein said electro-optic element further includes a reflector carried on the rear surface of said rear substrate.
 8. The electro-optic device as defined in claim 6, wherein said second electrode is reflective.
 9. The electro-optic device as defined in claim 6, wherein said electrochromic medium is a solution-phase electrochromic medium.
 10. An external electrochromic rearview mirror for a vehicle comprising: front and rear elements each having front and rear surfaces and being sealably bonded in spaced-apart relation to form a chamber; a transparent first electrode carried on one of said front surface of said rear element and said rear surface of said front element; a second electrode carried on one of said front surface of said rear element and said rear surface of said front element, wherein either said second electrode is reflective or a separate reflector is provided on a surface of said rear element; an electrochromic medium contained in said chamber in electrical contact with said first and second electrodes; a first layer having a high refractive index disposed in front of said front surface of said front element; a second layer having a low refractive index disposed on said first layer; a third layer of a photocatalytic material disposed on said second layer; and a fourth layer of a hydrophilic material disposed on said third layer.
 11. The external electrochromic rearview mirror as defined in claim 10, wherein said third layer includes titanium dioxide.
 12. The external electrochromic rearview mirror as defined in claim 11, wherein said fourth layer includes silicon dioxide.
 13. The external electrochromic rearview mirror as defined in claim 10, wherein said fourth layer includes silicon dioxide.
 14. The external electrochromic rearview mirror as defined in claim 10, wherein said first layer includes titanium dioxide.
 15. The external electrochromic rearview mirror as defined in claim 14, wherein said second layer includes silicon dioxide.
 16. The external electrochromic rearview mirror as defined in claim 10, wherein said second layer includes silicon dioxide.
 17. The external electrochromic rearview mirror as defined in claim 10, wherein said fourth layer has a thickness of less than about 800 Å.
 18. The external electrochromic rearview mirror as defined in claim 10, wherein said rearview mirror is non-planar. 